The field of this invention relates to slurries of abrasive inorganic oxide particles. In particular, it relates to slurries of inorganic oxide particles used to planarize or polish electronic chips, especially chips containing conductive metal circuits and silica-based insulating layers. Copper is increasingly being used as a conductive layer, and tetraethoxysilane (TEOS) dielectric is widely used as an insulating layer in such circuits.
The process employing these abrasive slurries is known as a chemical/mechanical planarization (or polishing) process, also known as xe2x80x9cCMPxe2x80x9d. Mechanical polishing is imparted by the abrasivity of the inorganic oxide particles in the slurry and chemical additives included in the slurry impart an additional effect of facilitating dissolution and removal of the surface being polished.
Electronic chips are polished or planarized because conductive and/or insulating layers are applied in excess during a series of steps needed to create the final circuit of the chip. The damascene process for making electronic chips is an example of when such polishing is used. Briefly, the damascene process applies excess copper onto an insulating layer containing channels that correlate with a desired circuit. Copper fills those channels as well as covers the insulating layer. This excess on the insulating layer, or the so-called xe2x80x9coverburden,xe2x80x9d has to be removed by polishing. It is desired to polish the deposited copper layer in such a manner that the overburden is completely removed before the next layer of material is applied. The additional layers are typically applied by photolithography and the underlying layers need to be sufficiently planarized to maximize the sharpness of focus in the subsequent photolithography steps.
The slurry also must provide uniform polishing across the wafer without undue scratching or pitting of the polished substrate. In conjunction with meeting this requirement, it is also further desirable to maximize polish rates in order to maximize the productivity of high-cost polishing equipment.
The conductive layer must also be removed during the damascene process with minimal xe2x80x9cdishingxe2x80x9d (see FIGS 1A, 1B and 1C). Dishing can occur during the layer deposition process (FIG. 1A) or as the polishing process reaches the insulating layer and is caused by the conductive layer being removed at a faster rate than the adjacent insulating layer (FIG. 1B). For example, slurries used in CMP processes typically comprise fine sized, i.e., submicron, inorganic abrasive particles. In particular, micronized amorphous silica particles have proven utility in CMP slurries based on their good colloidal stability and uniform polishing with minimal scratching. However, these abrasives generally do not yield equal polishing rates when employed in acidic, oxidizing slurries for polishing chips containing copper. Specifically, slurries prepared with these abrasives impart significantly lower polish rates for a silica-containing insulating layer compared to a copper-containing conducting layer. As a result, copper is removed more quickly, and significant (and undesirable) dishing can occur unless the polishing process is stopped at exactly the moment that the polishing process exposes the insulating layer. By contrast, a slurry with equal polish rates for copper and insulating layer will result in a planar surface even when polishing is continued briefly after the dielectric layer is exposed (FIG. 1C). FIGS. 1A, 1B and 1C also illustrate the use of a barrier layer. A barrier layer is a protective layer applied to the surface of the conductive layer in order to limit diffusion of metal, e.g., copper, into the dielectric layer during chip processing. The presence of a barrier may also play a part in dishing if the barrier polishes at a significantly slower rate than the copper. It alone may even play a part in creating dishing.
As a result of the different removal rates of the above materials, CMP processes using known abrasive slurries generally will not impart uniform polishing across the wafer. Therefore, it is generally desirable, e.g., in a damascene process, to employ a first abrasive slurry to remove most of the copper overburden. These first slurries are primarily effective because they contain aggressive chemical additives, e.g., glycine and hydrogen peroxide mixtures, which accelerate the removal of copper. There are some issues as to when polishing with this first slurry should be stopped. Suffice it to say, many operators stop the process when the barrier layer is first exposed. After polishing with the aggressive slurry, a second slurry which does not contain glycine is employed to finish the polishing on a finer level to ensure copper is completely removed from every location on the chip outside of the conducting line.
Either of the two slurries above, however, tend to cause the dishing effect described above. At least some dishing occurs when the polishing using the first slurry is carried out until the barrier layer is reached and that dishing remains or is even amplified after the second polishing step.
In addition, new insulating and barrier materials are frequently being developed. These new materials will typically have different properties and as a result will show different polishing rates. Accordingly, when these materials are introduced, the operator of the process will either need to adjust the abrasivity of the existing polishing slurry, or completely replace the existing slurry system with another having the appropriate abrasivity. It would be more desirable to adjust the existing slurry than to find a replacement slurry. However, it has been found that modifying conventional slurries for polishing conductive surfaces, e.g., copper, has not resulted in the selectivity desired. Alumina slurries, and fumed silica slurries, have been used in the past to polish copper surfaces.
U.S. Pat. No. 5,527,423 to Neville, et al. discloses examples of such slurries. The ""423 patent to Neville et al. discloses CMP slurries comprising fumed silicas or fumed alumina particles dispersed in a stable aqueous medium. Neville also mentions that precipitated alumina can be used. Neville et al. disclose that the particles have a surface area ranging from about 40 m2/g to about 430 m2/g, an aggregate size distribution less than about 1.0 micron and a mean aggregate diameter less than about 0.4 microns. This patent also discusses references that teach the addition of agents, such as hydrogen peroxide, or alkaline materials to CMP slurries. Other patents that disclose CMP slurries containing hydrogen peroxide and/or other acidic or alkaline additives include U.S. Pat. No. 5,700,838 to Feller, et al., U.S. Pat. No. 5,769,689 to Cossaboon, et al., U.S. Pat. No. 5,800,577 to Kidd and U.S. Pat. No. 3,527,028 to Oswald. In general, slurries such as these are based on the concept of selecting an inorganic oxide particle and either relying on the particles"" inherent abrasive properties for polishing or by including additional additives to the slurry in order to adjust the abrasive and/or polishing effects imparted by the slurry.
U.S. Pat. No. 4,304,575 to Payne discloses the preparation of aqueous silica sols for use as abrasive materials in mechanically polishing semi-conductor wafers. Payne""s method for preparing the sol comprises heating an initial alkaline aqueous silica sol containing a mixture of relatively smaller particles and relatively larger particles. It is stated by Payne that the smaller particles dissolve and redeposit on larger particles thereby producing an aqueous silica sol in which the majority of the silica particles have a size significantly larger than the larger silica particles in the starting mixed sol. Payne""s materials are prepared from sols having average particle size less than 100 millimicrons and preferably having final particle size of about 180 millimicrons. A similar disclosure is set forth in U.S. Pat. No. 4,356,107 also to Payne.
It is still desirable to find abrasive slurries which provide relatively equal selectivity among copper and the other various layers used to make electronic chips. It is also desirous to devise methods of making abrasive slurries in such a way that the abrasiveness of the particles can be easily adjusted to meet the polishing requirements at hand without having to resort to modifying the chemical makeup of the slurry or a new starting material for the abrasive particle.
In this invention, oxidizing agents are combined with slurries of fine, porous, inorganic oxide particles which have been prepared by heating the particles, e.g., in an autoclave, to modify and/or increase the particles"" abrasivity. These slurries preferably have a median particle size in the range of 0.1 to about 0.5 microns, and substantially all of the particle size distribution is below one micron. Slurries produced by this heating process (without oxidizing agent) have abrasive properties such that an alkaline slurry (e.g., at pH 10.8) consisting of water and the inorganic oxide particles removes silica at a rate of at least 120 mm/minute at 200 psixc2x7rpm. This measurement was made at a solids content of 12.6% by weight, at a pH of about 10.8 and with a Strasbaugh 6CA polisher with a SUBA 500 pad at a two minute polish time. The oxidizing agents added to these slurries include those known in the art, e.g. hydrogen peroxide. It has also been found that when oxidizing agent is added to slurries of inorganic oxide particles prepared in this fashion, and preferably the pH of the slurry is adjusted appropriately, the resulting slurry polishes copper at a rate which is relatively equal to its rate of polishing conventional insulating and barrier layers.
As mentioned above, autoclaving slurries of the above-mentioned porous particles imparts an increased abrasiveness to the particles. This is reflected in increased removal rates of silica substrate at standard polishing conditions. This increase in particle abrasivity strongly correlates with a decrease in particle surface area as determined by N2 adsorption (BET method). Such a correlation can be used for providing methods of simply modifying the abrasivity of the slurry. It is thought that this increase in particle abrasiveness and associated decrease in particle surface area is attributable to silica transport during the autoclaving process whereby silica is preferentially dissolved from sharply convex surfaces within the porous particle and redeposited at sharply concave surfaces at the junction of silica subunits (ultimate particles) that make up the porous particle. This A redeposition should thus strengthen the porous silica particle and increase its abrasivity. The elevated temperatures associated with autoclaving serve to accelerate this dissolution-redeposition process by increasing silica solubility. A similar process takes place in alkaline aqueous suspensions of silica particles held at room temperature or temperatures up to ambient pressure boiling (xcx9c100xc2x0 C.), but much longer times would be required. Therefore abrasivity of the particles in this invention can be modified over a wide range of properties by modifying the heating conditions used to reposition the inorganic oxide within the particles"" pore structure. Accordingly, the abrasivity of the particles can be adjusted as new insulating materials are being combined with copper conductive layers.